Diverticulitis
- Diverticula are little outpouchings of colon wall — tiny pockets that balloon out where the wall is weakest. Diverticulitis is when one of those pockets gets inflamed.
- The classic patient is older with left-lower-quadrant pain, because diverticula love the sigmoid colon.
- CT is the workhorse: look for a segment of bowel wall thickening, diverticula, and — the giveaway — dirty, streaky fat right next to it.
- Your main job is sorting uncomplicated (just inflammation) from complicated (abscess, free air, obstruction, or fistula) — that split changes the whole treatment plan.
Imagine an old bicycle inner tube. Over years of pressure, weak spots in the rubber bulge out into little blisters. The colon does the same thing: where blood vessels pierce the muscular wall, the lining can herniate outward into small pouches called diverticula. Most of the time they just sit there, minding their own business, and the person never knows. Diverticulitis is the day one of those pouches throws a tantrum.
What's actually going on
The leading idea is mechanical: a pouch gets obstructed — a little fecal plug, some local irritation — pressure builds, the wall gets inflamed and microscopically perforates, and the body walls it off. Think of a clogged drainpipe junction that starts leaking and swelling around the seal. That inflammation spilling into the surrounding fat is the thing imaging is built to catch.
Geography matters. In Western populations diverticula cluster in the sigmoid colon — the S-shaped segment in the left lower quadrant — so that's where the pain usually lands. (It's worth knowing that right-sided diverticulitis is more common in some Asian populations and loves to masquerade as appendicitis. Same neighborhood, different villain.)
Why we reach for CT
You can see complications on a plain film, but a plain abdominal radiograph is mostly there to rule out the scary stuff like free air. The real diagnosis lives on a contrast-enhanced abdominal CT, which shows the bowel wall, the pouches, and the surrounding fat all at once.
Here's the mental checklist I run:
| Sign | What it looks like | Why it matters |
|---|---|---|
| Diverticula | Small round outpouchings off the colon wall | You can't have diverticulitis without diverticula — confirm they're there |
| Wall thickening | A focal segment of colon looks plump | Marks the inflamed segment |
| Fat stranding | Hazy, streaky, "dirty" fat hugging that segment | The single most useful sign — clean fat is black, inflamed fat looks smudged |
| Engorged vessels | Prominent vessels feeding the area | Confirms an active inflammatory process |
That fat stranding is the one to fall in love with. Normal fat on CT is calm and uniformly dark. Inflamed fat looks like someone smeared it with a thumb — wispy gray streaks cutting through the black. Find a thickened sigmoid segment wrapped in smudgy fat, with diverticula in the wall, and you've basically made the call.
Uncomplicated vs complicated — the only split that matters
Most diverticulitis is uncomplicated: wall thickening and fat stranding, and that's it. It often gets managed conservatively. The work that earns your keep is hunting for complications, because any one of them can change the plan toward drainage, antibiotics with a longer leash, or surgery.
Always go looking for the complications even when the inflammation looks mild — an abscess or a tiny bubble of free air can hide next to an unimpressive-looking segment.
The complications to scan for:
| Complication | What to look for |
|---|---|
| Abscess | A walled-off fluid collection, often with a rim and sometimes gas inside |
| Perforation / free air | Tiny gas bubbles outside the bowel near the inflamed segment, or frank pneumoperitoneum |
| Fistula | An abnormal tract to another organ — classically the bladder, hinted at by air in the bladder with no recent catheter |
| Obstruction | A narrowed segment causing upstream bowel obstruction |
A small amount of localized gas can be a sealed micro-perforation that's managed conservatively, while a large volume of free air is a surgical emergency — so quantify what you see rather than just saying "free air."
The big mimic to keep honest about
A thickened, stranded sigmoid segment in diverticulitis can look uncomfortably like colon cancer — both thicken the wall and can drag in surrounding fat. Inflammation tends to involve a longer segment with lots of stranding relative to the wall change; cancer tends to be a shorter, more "shouldered" mass with enlarged nodes. The honest move is to recommend colonoscopy after the acute episode settles to make sure you weren't fooled.
That caveat isn't optional. The two can coexist, and an inflamed colon is an unreliable narrator, so the follow-up scope is part of doing this well.
The one thing to remember
Diverticulitis is "inflamed colon pouch, with dirty fat next door." See it, confirm the diverticula, then spend your real effort deciding whether it's uncomplicated or complicated — that's the read that changes what happens to the patient.